Exhaust gases discharged from diesel engines contain particulate matter (PM) comprising as main components carbonaceous soot and soluble organic fractions (SOFs) comprising high-boiling-point hydrocarbons, which are likely to adversely affect humans and environment when discharged into the air. Accordingly, ceramic honeycomb filters for capturing PM have conventionally been attached to exhaust pipes of diesel engines.
An example of ceramic honeycomb filters for capturing PM in exhaust gases to clean them is shown in FIGS. 1 and 2. The ceramic honeycomb filter 10 comprises a ceramic honeycomb structure comprising porous cell walls 2 defining a large number of outlet-side-plugged flow paths 3 and inlet-side-plugged flow paths 4, and a peripheral wall 1; and upstream-side plugs 6a and downstream-side plugs 6c sealing the exhaust-gas-inlet-side end surface 8 and exhaust-gas-outlet-side end surface 9 of the outlet-side-plugged flow paths 3 and the inlet-side-plugged flow paths 4 alternately in a checkerboard pattern. The peripheral wall 1 of the ceramic honeycomb filter is fixed by grip members (not shown) of metal meshes or ceramic mats, etc. to prevent movement during operation, and disposed in a metal container (not shown).
In the ceramic honeycomb filter 10, an exhaust gas is cleaned as follows. As shown by dotted arrows in FIG. 2, an exhaust gas flows into the outlet-side-plugged flow paths 3 open on the exhaust-gas-inlet-side end surface 8. While passing through the cell walls 2, particularly through communicating pores on and in the cell walls 2, PM is captured, cleaning the exhaust gas. The cleaned exhaust gas flows from the inlet-side-plugged flow paths 4 open on the exhaust-gas-outlet-side end surface 9 to the air.
PM continuously captured by the cell walls 2 clogs communicating pores on and in the cell walls, resulting in increased pressure loss when the exhaust gas passes through the ceramic honeycomb filter. Accordingly, PM should be burned off to regenerate the ceramic honeycomb filter before the pressure loss reaches a predetermined level.
The ceramic honeycomb filter should meet the requirements of a high capturing ratio of particulate matter and low pressure loss. However, because these requirements are in a contradictory relation, the optimization of porosity, pore volume, pores on the cell wall surface, etc. to meet both requirements has conventionally been investigated.
Further, to meet recent stricter exhaust gas regulations, investigations have been conducted to provide exhaust-gas-cleaning apparatuses comprising both SCR apparatuses for removing NOx and honeycomb filters for removing particulate matter. Thus, the honeycomb filters are required to have higher pressure loss characteristics than ever.
PM comprises numerous so-called nano-particles having diameters of 50 nm or less. Nano-particles are more accumulated in respiratory organs than larger particles having the same mass when inhaled. Also, because nano-particles have larger surface areas per volume, they become more harmful when toxic chemical substance is adsorbed to their surfaces. Because the mass of nano-particles contained in PM is small, a current PM mass limit is insufficient, and it is expected to implement a particle number limit for suppressing the emission of nano-particles largely affecting the number of particles discharged, as a future exhaust gas regulation. Accordingly, the honeycomb filters are required to have not only excellent pressure loss characteristics, but also an improved capturing ratio of PM, particularly an improved number-based capturing ratio of nano-particles, instead of the current mass-based capturing ratio of PM.
JP 2005-530616 A discloses a ceramic filter constituted by a cordierite honeycomb structure with ends plugged for capturing and burning particulate matter discharged from diesel engines, d50/(d50+d90) determined from a pore diameter distribution being less than 0.70, a permeability factor Sf when soot is accumulated, which is defined by the formula of [d50/(d50+d90)]/[porosity (%)/100] being less than 1.55, and a thermal expansion coefficient (25° C. to 800° C.) being 17×10−7/° C. or less, describing that with such a pore structure (pore diameter distribution and pore communication), low pressure loss can be kept even when PM is accumulated.
JP 2002-219319 A discloses a porous honeycomb filter made of cordierite as a main crystal phase, which has a controlled pore diameter distribution, the pore diameter distribution being such that the volume of pores having pore diameters of less than 10 μm is 15% or less of the total pore volume, the volume of pores having pore diameters of 10-50 μm is 75% or more of the total pore volume, and the volume of pores having pore diameters exceeding 50 μm is 10% or less of the total pore volume. JP 2002-219319 A describes that because of the above pore diameter distribution, this porous honeycomb filter has high efficiency of capturing PM, etc., with suppressed pressure loss increase due to the clogging of pores. JP 2002-219319 A also describes that such pore diameter distribution can be controlled by adjusting the particle diameters of a silica component, one of cordierite-forming materials, and by lowering the concentration of kaolin.
JP 2003-40687 A discloses a ceramic honeycomb structure composed of cordierite as a main component, and having porosity of 55-65% and an average pore diameter of 15-30 μm, the total area of pores open on the cell wall surfaces being 35% or more of the total cell wall surface area. JP 2003-40687 A describes that this ceramic honeycomb structure exhibits high capturing efficiency with low pressure loss.
Though the exhaust-gas-cleaning filters described in JP 2005-530616 A, JP 2002-219319 A and JP 2003-40687 A exhibit relatively high PM-capturing performance by the accumulation of PM, such PM-capturing performance is not necessarily sufficient at an early stage of use before PM is accumulated (when the ceramic honeycomb filters start to be freshly used or reused after regeneration). Particularly, they have insufficient capturing efficiency of nano-sized PM thought to be harmful by the stricter exhaust gas regulations, causing the problem that uncaptured harmful nano-sized PM is released.
JP 2004-360654 A discloses a ceramic honeycomb filter whose cell walls have porosity of 55-75% and an average pore diameter of 15-40 μm, the total area of pores open on the cell wall surface being 10-30% of the total cell wall surface area, and the number of pores having equivalent circle diameters of 5-20 μm being 300/mm2 or more among those open on the cell wall surfaces. However, even the ceramic honeycomb filter described in JP 2004-360654 A fails to effectively capture nano-particles at an early stage of use before PM is accumulated, though a mass-based capturing ratio of PM is improved to some extent. Namely, it has a low number-based capturing efficiency of PM, less expected to meet the particle number limit.
WO 2011/102487 A1 discloses a ceramic honeycomb structure comprising cell walls having (a) porosity of 55-80%, and (b) a median pore diameter d50 of 5-27 μm when measured by mercury porosimetry; (c) an area ratio of pores open on the surface being 20% or more; (d) an area-based median pore opening diameter d50 (expressed by an equivalent circle diameter) of pores open on the surface being 10-45 μm; (e) the density of pores open on the surface, which have equivalent circle diameters of 10 μm or more and less than 40 μm, being 350/mm2 or more; (f) the maximum inclination of a curve of a cumulative pore volume to a pore diameter, which is obtained from the pore diameter distribution measured by mercury porosimetry, being 1.6 or more; and (g) a ratio D50/d50 of the median pore diameter d50 and the median pore opening diameter d50 being 0.65 or less. WO 2011/102487 A1 describes that a ceramic honeycomb filter constituted by this ceramic honeycomb structure can effectively capture nano-particles largely affecting the number of particles discharged even at an early stage of use before PM is accumulated, resulting in an improved number-based capturing ratio of PM, with less deterioration of pressure loss characteristics after captured PM is accumulated.
However, when a ceramic honeycomb filter constituted by the ceramic honeycomb structure described in WO 2011/102487 A1 is used as an exhaust gas filter for diesel engine cars, a capturing ratio of namo-sized PM may be insufficient in a driving mode, in which driving and stopping are repeated on city roads, etc. Thus, further improvement of the number-based capturing ratio of PM is desired to meet increasingly stricter exhaust gas regulations.
JP 2009-517327 A discloses a porous cordierite honeycomb having increased mechanical strength and heat shock resistance, which has an average CTE of 9×10−7/° C. or less between 25° C. and 800° C., and meets MA<2220, and MT>2660, wherein MA=3645 (IA)−106 (CTE)+19 (d90)+17 (porosity %), and MT=4711 (IT)+116 (CTE)−26 (d90)−28 (porosity %), IA is an I ratio of XRD measured in a longitudinal cross section of the honeycomb, and IT is an I ratio of XRD measured on cell wall surfaces of the honeycomb. JP 2009-517327 A describes that the porous honeycomb preferably has porosity of 40% or more and less than 54%, and a median pore diameter of 10 μm or more.
JP 2011-516371 A discloses a porous ceramic body formed by a polycrystalline ceramic having an anisotropic fine structure, which is constituted by oriented polycrystalline networks (reticular formations) having an anisotropic factor Af-pore-long meeting 1.2<Af-pore-long<5. JP 2011-516371 A describes that the ceramic body has a narrow pore diameter distribution and porosity of more than 50%, a median pore diameter being in a range of 12-25 μm. It describes that this ceramic body exhibiting high strength, a low thermal expansion coefficient (CTE), and high porosity can be used for automobile substrates, particulate-removing filters for diesel engines and gasoline engines, functional filters such as catalyst filters having a partial or complete addition of NOx, etc.
WO 2011/027837 A1 discloses a ceramic honeycomb structure comprising cell walls having porosity of 40-60%; an opening area ratio of pores open on the cell wall surfaces (the total area of pore openings per a unit area of cell wall surfaces) being 15% or more; an area-based median opening diameter of pores open on the cell wall surfaces, which is expressed by an equivalent circle diameter (diameter of a circle having the same area as the opening area of each pore), being 10 μm or more and less than 40 μm; the density of pores having equivalent circle diameters of 10 μm or more and less than 40 μm being 350/mm2 or more; and the average circularity of pores having equivalent circle diameters of 10 μm or more and less than 40 μm being 1-2. WO 2011/027837 A1 describes that the ceramic honeycomb structure has an improved PM-capturing ratio with low pressure loss at an early stage after regeneration, thereby enabling the efficient capturing of namo-sized PM, which has been increasingly needed because of stricter exhaust gas regulations.
WO 2007/108428 A1 discloses a method for producing a honeycomb structure by using an alumina source, a silica source, and a magnesia source each having a 50-volume-% particle diameter V50 (μm) of 1-25 μm in a volume-based particle diameter distribution; and using a cordierite-forming material having a volume-based particle diameter ratio (Vall90/Vall10) of 10 or less, wherein Vall10 is a particle diameter at 10% by volume, and Vall90 is a particle diameter at 90% by volume, in a volume-based particle diameter distribution of the entire cordierite-forming material; and the difference between Vall90 and Vall10 [width of a volume-based particle diameter distribution (Vall90−Vall10)] being 25 m or less. WO 2007/108428 A1 describes that a honeycomb structure obtained has high porosity and a sharp pore diameter distribution, useful for exhaust-gas-capturing filters, particularly for diesel particulate filters (DPF) for capturing particulate matter, etc. in exhaust gases discharged from diesel engines.
However, when the honeycombs described in JP 2009-517327 A, JP 2011-516371 A, WO 2011/027837 A1 and WO 2007/108428 A1 are used for exhaust-gas-cleaning filters, their PM-capturing performance is not necessarily sufficient at an early stage of use before PM is accumulated (when ceramic honeycomb filters start to be freshly used or reused after regeneration), though it becomes high as PM is accumulated to some extent. Particularly the capturing efficiency of namo-sized PM, which has become important due to stricter exhaust gas regulations, is insufficient, failing to capture harmful namo-sized PM.